Method for surface densification and calibration of a sintered component

Abstract

Method for surface densification and calibration of a sintered component (2), wherein the sintered component (2) is moved along an axis (3) from a first die opening (6) towards a second die opening (13) of a die tool (1) opposite the first die opening (6) along the axis (3), wherein the sintered component (2) passes through several die sections (7-11) of the die tool (1) during this movement and a surface area of ​​the sintered component (2) is densified, for which purpose an inner diameter (17) of the successive die sections (7-11) decreases in the pressing direction and the individual die sections (7-11) are arranged such that a subsequent die section (8-11) of the several die sections (7-11) each connects directly to the corresponding preceding die section (7-10) in the pressing direction.and that, after surface compaction in a matrix section (11) with a decreasing inner diameter (17) following the last matrix section (11), at least nearly complete stress relief of the sintered component (2) is carried out in a relief section (21) immediately adjoining the last matrix section (11), which has an inner diameter (22) that is at least 0.02% larger than that of the last matrix section (11) formed immediately before it and a smaller inner diameter (22) than that of the matrix section (7-11) with a decreasing inner diameter (17), and that the sintered component (2) is calibrated in the relief section (21), for which purpose the inner contour of this relief section (21) corresponds to the target contour with target dimension of the sintered component (2), characterized in thatthat the inner contour of the penultimate die section (10) of the sequence of die sections (7-11) with decreasing inner diameter (17) corresponds with respect to the geometric dimensions in the direction perpendicular to the pressing direction of the inner contour of the relief section (21) with the target contour having the nominal dimension.

Description

The invention relates to a method for surface densification and calibration of a sintered component, in which the sintered component is moved along an axis from a first die opening towards a second die opening of a die tool opposite the first die opening along the axis, wherein the sintered component passes through several die sections of the die tool during this movement and a surface area of ​​the sintered component is densified, for which purpose an inner diameter of the successive die sections becomes smaller in the pressing direction and the individual die sections are arranged such that a subsequent die section of the several die sections immediately connects to the corresponding preceding die section in the pressing direction.and that after surface compaction in a relief section immediately following the last die section with decreasing inner diameter, the sintered component is relieved in a relief section immediately following the last die section, which has an inner diameter that is at least 0.02% larger than that of the immediately preceding last die section and smaller than that of the first die opening, and that the sintered component is calibrated in the relief section, for which purpose the inner contour of this relief section corresponds to the target contour with target dimension of the sintered component. Sintered components, i.e., workpieces made from pressed and sintered metal powder, have long been an alternative to cast or machined workpieces. However, the porosity of sintered components, which is inherent in the manufacturing process and varies in degree, negatively impacts their mechanical properties, thus limiting their use. Various methods for reducing surface porosity are known from the prior art. For example, rotationally symmetrical sintered components are often rolled. From JP H10-085 995 A, a method for compacting a sintered component using a die tool is known. The die workpiece has several die sections that are directly adjacent to one another, with the inner diameters of the die sections decreasing in the pressing direction of the sintered component by the die tool. A similar procedure is known from RU 2 156 179 C2. From EP 2 066 468 A2, a method for surface densification of a sintered component is known, in which the sintered component is moved in a die tool along an axis in a pressing direction through several die sections from a first die section at a first die opening to a last die section, wherein a wall surface of each die section forms at least one pressing surface against which a contact surface formed by an outer surface of the sintered component is pressed, and an inner contour defined by the pressing surface, lying in a cross-section with respect to the axis, corresponds at least approximately to an outer contour defined by the contact surface.During the movement of the sintered component from the first die opening to the last die section, surface compaction occurs through continuously transitioning die sections and monotonically decreasing inner diameters of the die sections measured between interacting press surfaces. In the process described in the latter EP-A2, calibration of the sintered component can optionally be performed after surface densification. For this purpose, a subsequent calibration section is provided after the last die section, which has a calibration diameter corresponding to a nominal diameter of the sintered component on its outer surface. The calibration section can connect directly to the last die section, i.e., the second, lower die opening, or it can be provided with a gap between the last die section and the dimensionally accurate calibration section, thus allowing for intermediate stress relief of the sintered component before calibration. It is further described that the calibration section includes a calibration plate that rests against the second, opposite tool surface.The calibration of the sintered component can be performed either immediately after the final surface compaction or with the insertion of a relief section. The relief section connects directly to the second die opening. US 5,631,029 A describes a cold isostat press consisting of a press body with a cavity open at at least one end, an elastic mold for compacting metal powder positioned in the cavity, means functionally connected to the mold for expanding the mold and compacting the metal powder, and at least one punch with a base section perpendicular to the cavity and an extension, wherein the extension has one of several geometric shapes and a surface in contact with the metal powder, the surface having a first and a second section substantially perpendicular to the cavity, and wherein the first section extends further into the cavity than the second section to reduce flared ends of a press-formed compact. The object of the invention is to provide a simplified method for surface densification of a sintered component. The object of the invention is solved by the method mentioned at the outset, in which the inner contour of the penultimate matrix section of the sequence of matrix sections with decreasing inner diameter corresponds with respect to the geometric dimensions in the direction perpendicular to the pressing direction of the inner contour of the relief section with the target contour having the target dimension. An advantage is that no further forming of the sintered component from its unloaded state occurs before calibration, thus reducing burr formation on the sintered component caused by the kneading effect during surface compaction. Furthermore, this also reduces the mechanical stress on the die tool, as further compaction of the sintered component from its unloaded state requires higher forming forces after it has already been surface compacted in previous compaction steps. Combining the calibration and unloading sections also shortens the process time for surface compaction and calibration of the sintered component. The inner contour of the penultimate die section in the sequence of die sections with decreasing inner diameter corresponds, with respect to its geometric dimensions perpendicular to the pressing direction, to the inner contour of the die section with the nominal dimension. This is particularly advantageous when the sintered part is removed through the first die opening through which it was inserted into the die. This ensures that the sintered part passes through a calibration section three times during its production. The sintered part is first compacted to the nominal dimension in the aforementioned penultimate die section. In the subsequent final die section with decreasing inner diameter, it is compacted once more before entering another calibration section, where it is simultaneously decompressed.After the motion reversal, the sintered component passes through the aforementioned last die section again and is calibrated once more in the penultimate die section. This improves the component accuracy. In a preferred embodiment of the method, a die tool is used in which the relief section is formed. Thus, a one-piece die tool is preferably used for both surface compaction and calibration of the sintered component. On the one hand, this reduces the setup time of the compaction and calibration press, since alignment of the die tool with the calibration plate, as required in the prior art, is no longer necessary. On the other hand, this also increases component accuracy. Due to the one-piece design of this tool, it can also withstand higher loads, and defects in the transition of the sintered component from the die tool to the calibration plate, which can occur with tools according to the prior art, are avoided. It is also possible that, after calibration, the sintered component is moved again against the pressing direction through the last of the die sections with a decreasing inner diameter. This can further increase the accuracy of the sintered component. In a further embodiment of the process, the sintered component can be provided with a first edge and a second edge opposite it in the pressing direction. These edges are formed at transitions between an end face that can be applied to the die sections and the axial end faces of the sintered component. The first and / or the second edge is / are beveled before being inserted into the die tool. This improves the insertion of the sintered component into the die tool because the beveling results in less shear at the edges of the sintered component. This reduces the risk of breakage during insertion of the sintered component into the die tool.Furthermore, it was observed that an improvement in the "cylindrical geometry" can be achieved with (nearly) cylindrical components, such as gears, meaning that the sintered parts also exhibit higher component accuracy. This design variant also counteracts burr formation in the edge area. This, in turn, reduces the manufacturing effort for the sintered component, as subsequent burr removal is simpler or even unnecessary. Such burrs on sintered components can lead to damage to other (sintered) components in contact with them, especially if the sintered components are intended for rotary motion. In addition to these effects, this design variant can also increase the load-bearing capacity of the sintered component by reducing edge bearing. To further improve these effects, one design variant provides for the first edge, which is positioned above the second edge during surface compaction and calibration of the sintered component, to be more deeply faceted than the second edge. This allows for more space in the upper area of ​​the sintered component (in the pressing direction) for material displacement from areas of the sintered component located below it (in the pressing direction). To better understand the invention, it is explained in more detail with reference to the following figures. The figures show, in simplified, schematic representations: Fig. 1 a section through a cutout from a die tool with a sintered component shortly before the insertion position; Fig. 2 the section through the cutout from the die tool according to Fig. 1 with the sintered component in the calibration position; Fig. 3 a section through a tool for faceting the sintered component; Fig. 4 a schematic comparison of the state of the sintered component after sintering, after faceting, and after surface densification and calibration. It should be noted at the outset that in the differently described embodiments, identical parts are provided with the same reference numerals or component designations, and the disclosures contained in the entire description can be applied analogously to identical parts with the same reference numerals or component designations. Furthermore, the positional designations chosen in the description, such as top, bottom, side, etc., refer to the figure directly described and illustrated and, in the event of a change in position, must be applied analogously to the new position. It should be noted here that calibrating a sintered component refers to its machining in a tool using pressing forces to achieve at least an approximate approximation of the component's nominal dimensions. "At least an approximation" means that deviations from the nominal dimensions within the usual tolerances are permissible. In the context of the invention, the term "target dimension" refers to the final dimension that the finished sintered component 2 should have, optionally less the increase in size of the sintered component 2 after stress relief (i.e., ejection from the calibration die, as will be explained below), which is defined by the springback behavior of the sintered material due to elastic recovery. The proportion of springback behavior can be determined empirically. In other words, the target dimension plus any increase due to elastic recovery yields the final dimension. Figures 1 and 2 show a section of a die tool 1 for surface densification and calibration of a sintered component 2 in longitudinal section. The sintered component 2 consists of pressed and subsequently sintered powder metal, the processes and materials for producing such a sintered blank being sufficiently known from the prior art and therefore not being explained in more detail. For surface compaction and calibration, the sintered component 2 is moved along an axis 3 through the die tool 1. The die tool 1 comprises a tool body 4, which has a first (upper) die opening 6 on a tool surface 5, from which several die sections 7 to 11 extend along the axis 3 into the interior of the tool body 4. The first die section 7 adjoins the first die opening 6, while the last die section 11 is located closer to a second tool surface 12 opposite the first tool surface 5 along the axis and to a second die opening 13 formed therein. In the illustrated embodiment, the sintered component 2 is designed in a disc shape and has a diameter 15 on a radial outer surface 14, i.e. the end face, which corresponds to a raw diameter before surface compaction and to a smaller final diameter after surface compaction. Generally, rotationally symmetrical and / or at least approximately cylindrical sintered components 2, such as gears, etc., are preferably surface-compacted and calibrated using the die tool 1. However, other sintered components 2 can also be machined using the die tool 1. The surface densification of the sintered component 2 is achieved by inserting it through the first die opening 6 into the first die section 7 and subsequently moving it into all further die sections 8 to 11, whereby in each die section 7 to 11 the outer surface 14 of the sintered component 2 is pressed against wall surfaces 16 of the die sections 7 to 11, at least on sections of the outer surface 14. In this process, one or more contact surfaces on the outer surface 14 of the sintered component 2 come into pressure contact with one or more pressure surfaces on the wall surfaces 16 of the die sections 7 to 11. The contact surface can be formed by part or all of the outer surface 14 of the sintered component 2. The pressure surface can be formed by a partial section of the wall surface 16 or by the entire wall surface 16, whereby the partial section can refer to the axial extent and / or to the circumferential extent. The pressing effect is achieved by ensuring that the inner diameter 17 of the die sections 7 to 11, defined by the clear width between opposing or interacting sections of the pressing surface of a die section 7 to 11, is smaller than the diameter 15 of the sintered component 2 before it is inserted into the respective die section 7 to 11. Generally, the die sections 7 to 11 preferably have an inner contour that corresponds to the outer contour of the sintered component 2, but each die section 7 to 11 has a circumference that is smaller than the circumference of the sintered component 2 before it is inserted into the respective die section 7 to 11. The successive matrix sections 7 to 11 along axis 3 transition directly (continuously), i.e., without intermediate sections, into one another and have (monotonic) decreasing inner diameters 17 from the first matrix section 7 to the last matrix section 11, meaning that successive matrix sections 7 to 11 can be the same size or, in particular, become smaller, but not larger. As a result, the pressing force on the contact surface of the sintered component 2 increases from the first matrix section 7 to the last matrix section 11, thus defining a pressing direction along axis 3 that points from the first matrix section 7 to the last matrix section 11.The movement of the sintered component 2 in the die tool 1 preferably takes place in a straight line in this pressing direction from the first die opening 6 to the last die section 11, after which the demolding of the sintered component 2 from the die tool 1 preferably takes place after reversing the direction of movement against the pressing direction through the first die opening 6. A rotary motion can also be superimposed on the linear movement in the direction of axis 3, causing the sintered component 2 to perform a screwing movement in the die tool 1. The press fit between the aforementioned contact surfaces and the aforementioned pressing surfaces generates compressive stresses that are essentially oriented perpendicular to the contact surfaces. These stresses acting on the contact surfaces in the sintered component 2 cause both elastic and plastic deformation of the sintered component 2, with the plastic component resulting in permanent surface densification. During this surface densification, the powder metal particles, bonded together by pressing and subsequent sintering at so-called bridges, are strongly pressed against each other and plastically deformed. The pore-like cavities present between the powder metal particles after sintering are thereby reduced in volume, and the material density in this area is increased. The effect of surface densification is greatest directly at the contact surface and decreases towards the interior of the sintered component 2. Using this method, surface layers of sintered components 2 with a thickness ranging from a few hundredths of a millimeter to several tenths of a millimeter and beyond can typically be densified. The relative movement required for the process between the sintered component 2 and the die tool 1 can be achieved by moving the sintered component 2 and / or by moving the die tool 1, wherein the sintered component 2 and the die tool 1 are each connected to a suitable drive or a stationary frame. During surface compaction and subsequent calibration, the sintered component 2 is clamped between an upper punch 18 and a lower punch 19. For the downward movement, the upper punch 18 presses down on the sintered component 2 from above; the lower punch 19 can be pulled downwards or is also pushed downwards by the upper punch 18. For the preferred ejection of the sintered component 2 through the first die opening 6, the lower punch 19 is pushed upwards, and the upper punch 18 can optionally be pulled upwards.For these movements of the upper punch 18 and the lower punch 19, corresponding drives, not shown in detail, are provided. The transition from a die section 7 to 10 to the subsequent die section 8 to 11 can be formed as a chamfer 20 or with a radius, whereby a concave radius can be followed by a convex radius in the pressing direction. This allows for a smooth transition of the sintered component 2 from a die section 7 to 10 to the subsequent die section 8 to 11, without unintentional material removal from the sintered component 2 due to a sharp-edged step or chipping of the edges at the transitions of the die tool 1. As can be seen in Figures 1 and 2, such a chamfer can also be formed at the first die opening 6. The chamfers 20 or the respective radiuses are part of the respective die section 7 to 11 and therefore do not form intermediate sections. Although five die sections 7 to 11 are shown in the embodiment of the die tool 1 specifically depicted in Fig. 1 and Fig. 2, the die tool 1 can generally have between three and eight or more than eight such die sections. Since this design of the die tool 1 is known in principle from the aforementioned EP 2 066 468 A2, reference is made to it for further details. EP 2 066 468 A2, in this respect relating to surface compaction, is part of the present description. The last die section 11 shown in Fig. 1 is the die section of the die tool 1 that has the smallest inner diameter 17 or the smallest clear width. Immediately following this last die section 11 with the smallest inner diameter 17, a relief section 21 is provided or formed in the die tool 1. This relief section 21 has a larger inner diameter 22 compared to the last die section 11 formed immediately before it, which has a decreasing inner diameter 17. This allows the sintered component 2 to relax in this relief section 21. Simultaneously with this relaxation, the calibration of the sintered component 2 also takes place in the relief section 21. For this purpose, the relief section 21 has an inner contour that corresponds to the target contour with target dimension of the sintered component 2.The inner contour of the relief section 21 is therefore identical to the outer contour of the finished sintered component 2, both in terms of geometry and geometric dimensions (viewed in cross-section). This calibration position of the sintered component 2 is shown in Fig. 2. Following the relief section 21, the die tool 1 has a further section 23. This section 23 has an inner diameter 17, or a clear width, which corresponds to the inner diameter 17, or the clear width, of the last die section 11 with the smallest inner diameter 17. Section 23 serves to guide the lower punch 19 in the die tool 1. The inner diameter 22, or the clear opening of the relief section 21, corresponds to the outer diameter 15 (Fig. 1), or the clear opening of the finished sintered component 2. This inner diameter 22, or this clear opening of the relief section 21, is at least 0.02% larger, in particular between 0.02% and 0.1%, than the inner diameter 17, or the clear opening of the last die section 11 with the smallest inner diameter 17. However, the inner diameter 22, or the clear opening of the relief section 21, is not larger than the inner diameter, or the clear opening, of the first die opening 6. This is intended to enable at least nearly complete stress relief of the sintered component 2. As can be seen from Figures 1 and 2, the die tool 1 used is preferably designed as a single piece, so that it also includes the relief section 21. However, it is also possible that at least the relief section is formed by a separate, in particular plate-shaped tool, which is arranged directly adjacent to the die tool 1 for carrying out the process of surface densification and calibration of the sintered component 2. It is intended that the inner contour of the penultimate die section 10 of the sequence of die sections 7 to 11, with decreasing inner diameter 17, corresponds with respect to its geometric dimensions in the direction perpendicular to the pressing direction of the inner contour of the relief section 21, with the target contour having the nominal dimension. In other words, this penultimate die section 10, viewed in cross-section, is to be identical to the cross-section of the relief section 21 and thus to the calibration cross-section, both in terms of geometry and geometric dimensions in cross-section. According to one embodiment of the method, the sintered component 2 may have a first edge 24 and a second edge 25 opposite it in the pressing direction (as is common practice), which are formed at transitions between an end face 26 that can be applied to the die sections and axial end faces 27, 28 of the sintered component, and the first and / or the second edge is / are faceted before being inserted into the die tool. Figure 3 shows a longitudinal section of a press tool 29 with which such faceting can be produced by pressing. The press tool comprises a first lower press part 30 and a second upper press part 31. The first and second press parts 30, 31 have corresponding negative facets at the points where the edges 24 and 25 of the sintered part 2 come into contact. After sintering, the sintered part 2 is clamped between the first and second press parts 30, 31. By compressing these two press parts 30, 31 together through a predetermined stroke, the faceting is imparted to the sintered part 2 by material displacement. Fig. 4 shows a schematic state diagram of the sintered component 2. Line 32 shows the edge state after sintering, line 33 the edge state after machining in the press tool 29, and line 34 the edge state after surface densification and calibration of the sintered component 2 in the die tool 1 (Fig. 1). The faceting of the edges 24, 25 of the sintered component 2 is particularly pronounced as a rounding, as can be seen in Fig. 4. A maximum radius of curvature – the facets can have a radius of curvature that varies along their course, as can be seen in Fig. 4 – can be selected from a range of 0.1 mm to 5 mm. In principle, the first, upper edge 24 and the second, lower edge 25 of the sintered component 2 can be provided with identical facets. However, according to one embodiment, it is preferred that the first edge 24, which is positioned above the second edge 25 during the surface compaction and calibration of the sintered component 2, is more heavily faceted (i.e., formed with a larger facet per unit area) than the second edge 25. The method for surface densification and calibration of the sintered component 2 can also be applied to surface densification and calibration of openings, such as bores, in sintered components 2. Instead of the die tool 1, a punch is used, which, like the die tool 1, also has sections with different diameters and the corresponding calibration section in the stress-relieving stage. In this case, however, the diameter of the directly adjacent sections (monotonic) increases. All further descriptions of the die tool 1 also apply analogously to the punch, whereby the terms "inner" and "outer" must be changed accordingly. The exemplary embodiments show possible design variants of the die tool 1 and the press tool 29. Finally, for the sake of clarity, it should be noted that, for a better understanding of the structure of the die tool 1 or the press tool 29, some illustrations have been shown not to scale and / or enlarged and / or reduced in size. Reference symbol list 1 Die tool 2 Sintered component 3 Axis 4 Tool body 5 Tool surface 6 Die opening 7 Die section 8 Die section 9 Die section 10 Die section 11 Die section 12 Tool surface 13 Die opening 14 Outer surface 15 Diameter 16 Wall surfaces 17 Inner diameter 18 Upper punch 19 Lower punch 20 Chamfer 21 Relief section 22 Inner diameter 23 Section 24 Edge 25 Edge 26 End face 27 End face 28 End face 29 Press tool 30 Press part 31 Press part 32 Line 33 Line 34 Line

Claims

Method for surface densification and calibration of a sintered component (2), wherein the sintered component (2) is moved along an axis (3) from a first die opening (6) towards a second die opening (13) of a die tool (1) opposite the first die opening (6) along the axis (3), wherein the sintered component (2) passes through several die sections (7-11) of the die tool (1) during this movement and a surface area of ​​the sintered component (2) is densified, for which purpose an inner diameter (17) of the successive die sections (7-11) decreases in the pressing direction and the individual die sections (7-11) are arranged such that a subsequent die section (8-11) of the several die sections (7-11) each connects directly to the corresponding preceding die section (7-10) in the pressing direction.and that, after surface compaction in a matrix section (11) with a decreasing inner diameter (17) following the last matrix section (11), at least nearly complete stress relief of the sintered component (2) is carried out in a relief section (21) immediately following the last matrix section (11), which has an inner diameter (22) that is at least 0.02% larger than that of the last matrix section (11) formed immediately before it and a smaller inner diameter (22) than that of the matrix section (7-11) with a decreasing inner diameter (17), and that the sintered component (2) is calibrated in the relief section (21), for which purpose the inner contour of this relief section (21) corresponds to the target contour with target dimension of the sintered component (2), characterized in thatthat the inner contour of the penultimate die section (10) of the sequence of die sections (7-11) with decreasing inner diameter (17) corresponds with respect to the geometric dimensions in the direction perpendicular to the pressing direction of the inner contour of the relief section (21) with the target contour having the nominal dimension. Method according to claim 1, characterized in that a die tool (1) is used in which the relief section (21) is formed. Method according to claim 1, characterized in that the sintered component (2) is moved again through the last of the die sections (7-11) with decreasing inner diameter (17) after calibration against the pressing direction. Method according to one of claims 1 to 3, characterized in that the sintered component (2) has a first edge (24) and a second edge (25) opposite it in the pressing direction, which are formed at transitions between an end face (26) that can be applied to the die sections (7-11) and axial end faces (27, 28) of the sintered component (2), and that the first and / or the second edge (24, 25) is / are faceted before being inserted into the die tool (1). Method according to claim 4, characterized in that the first edge (24), which is arranged above the second edge (25) during surface compaction and calibration of the sintered component (2), is more faceted than the second edge (25).

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